1. Field of the Invention
The present invention relates to a tool electrode driving unit of an electric discharge machine, and in particular, to a driving unit including an electrode actuator for rapidly moving the tool electrode toward and away from a workpiece.
2. Description of the Related Art
During electric discharge machining, concentration of discharge and resulting state of continuous arc discharge occurs sometimes when the clearance between the workpiece and the tool electrode has become inappropriate. Such continuous arc discharge will cause conspicuous mark to be left on the spot of the workpiece where the concentration of discharge has occurred, thereby making defective the machined surface of the workpiece. In addition, the concentrated discharges cause machining fluid to be resolved to produce pyrolysis products. As a consequence, the tool electrode and the workpiece short-circuit each other through the pyrolysis products to hinder the further progress of the machining.
In order to avoid occurrences of such situation, it is known to rapidly move the tool electrode toward and away from the workpiece to forcedly introduce the machining fluid between the workpiece and the tool electrode, thereby preventing an occurrence of concentrated discharge. More particularly, occurrence of the continuous arc discharge can be prevented by a squeeze effect of the machining fluid.
Conventionally, in order to provide a mechanism for enabling rapid retreating and approaching of the tool electrode relative to the workpiece, the tool electrode is rigidly attached to the distal end of the quill (spindle) in the electric discharge machine through a piezoelectric element, which functions as an actuator to utilize strain caused by the piezoelectric element, thereby causing a jump motion of the tool electrode to occur. However, such a motion of the tool electrode by the piezoelectric element is an impactive rapid one, that is, high-acceleration motion, so that a large impactive reaction force acts on the portions at which the tool electrode is supported. For instance, a tool electrode having a weight of 50 kg in total will cause a force of about 1200 kgf to act on the quill or column when the piezoelectric element operates. This great reaction force causes the quill or column supporting the tool electrode to be bent to absorb and consume considerable part (about 25%) of displacement generated by the piezoelectric element to pull up the tool electrode. Therefore, according to the conventional arrangement, the displacement generated by the piezoelectric element cannot be fully utilized for generating a jump motion of the tool electrode. Besides, a force caused by an impactive motion and transferred to a head or column of the electric discharge machine causes damage to feed screws, etc., thereby shortening the service life of the machine.